Keywords

Notes

Acknowledgements

The author thanks the referee for the very detailed reading of the manuscript and numerous useful suggestions which permitted to simplify and shorten the proofs. In particular, the use of [6, Proposition 1.2.1] in the proofs of Lemmas 2.1 and 2.2, as well as the use of the order-preserving property of \(\tau _t\) in the proof of Lemma 2.6 are due to the referee. Section 4 also arose from referee’s comments.

This work was partially supported by FAPESP of Brazil (process number: 2012/01554–7).